HyreSolar tools
Usable kilowatt-hours for backup, for bill shifting, or for both.
Battery capacity is quoted in kilowatt-hours, but what you actually need depends on the load you want to carry and for how long. This converts that into usable energy, allowing for the round-trip efficiency losses that mean stored energy is always less than energy put in.
What this returns at the defaults
Carrying a 2 kW backup load for 12 hours at 90% round-trip efficiency needs 26.7 usable kilowatt-hours. Add 8 kWh of evening peak shifting and the requirement becomes 35.6 kWh. Note that only the product of load and hours matters: 1 kW for 24 hours and 3 kW for 8 hours both come to the same 26.7 kWh.
Last updated . Data as of 23 August 2026.
Usable kilowatt-hours for backup, shifting, or both.
Energy only. Surge rating is a separate check this model does not make.
HyreSolar does not sell batteries. Compare against usable capacity, not nameplate.
The battery sizing calculator returns usable energy, which is what you need to compare against a product’s usable capacity rather than its nameplate. Manufacturers publish both and they are not the same. Always compare like with like.
Energy is only half the specification, and this tool models the other half not at all. A battery has an energy rating in kilowatt-hours and a power rating in kilowatts, and the power rating is what decides whether it can start your loads. Our research found manufacturers publishing wildly different surge figures, 185 LRA on one product, "up to 48 A LRA" on another, and nothing at all on a third. The exclusion that bites in a real outage is surge, not energy.
Sizing for a very long outage gets expensive quickly, and there is a floor on run time nobody publishes. Across the products we read, usable capacity divided by the manufacturer’s own maximum continuous output comes to roughly 1.2 to 1.3 hours. That is what a battery does at full output; carrying a small load for a long time is the other end of the same trade.
Every input below is a number you can find, not one you have to guess. This is where each one comes from.
Not the house. A list of circuits. This is the single most important decision and it is made at quote time and then physically wired.
Where to find it Walk the house and write down what genuinely must run: refrigeration, some lighting, internet, a well pump, medical equipment, heating controls. Our critical load panel page explains why circuits move, not appliances.
The continuous draw of everything on the protected list running together, not the sum of every nameplate in the house.
Where to find it Appliance labels give watts. A fridge cycles rather than running constantly, so its average is well below its label. Be realistic: over-stating this input is the most common way people arrive at a battery they cannot afford.
How long an outage you are sizing for. Only the product of load and hours matters, so halving the load doubles the hours.
Where to find it Your own experience and your utility’s reliability record. Our outage statistics by state give the measured picture rather than an impression.
The kilowatt-hours you buy during expensive hours and could instead supply from storage charged earlier.
Where to find it A time-of-use bill shows consumption by period. If you are not on a time-varying tariff, leave this at zero, there is nothing to shift.
The share of energy put in that comes back out. Ninety percent is a reasonable planning figure; the datasheet gives the real one.
Where to find it The manufacturer’s datasheet states round-trip efficiency. Moving from 80% to 95% changes the requirement here from 30.0 kWh to 25.3.
Backup load in kW × hours, divided by round-trip efficiency.
Daily peak import in kWh, divided by round-trip efficiency.
The two add, because a battery cannot hold the same kilowatt-hour in reserve and spend it on arbitrage.
This tool does not do it. Compare your largest starting load against the product’s surge rating.
The formula, in full
backup usable kWh = (backup load kW × hours) ÷ round-trip efficiency. shift usable kWh = daily peak import kWh ÷ round-trip efficiency. both = backup + shift.
A household protecting refrigeration, lighting, internet, a gas furnace’s controls and a few outlets, around 2 kW continuous, through an overnight outage, on a time-of-use tariff with 8 kWh of evening peak import.
Inputs
Result
35.6 kWh usable
For both jobs at once. If you only want backup, 26.7 kWh. Note the losses: 24 kWh of delivered energy needs 26.7 kWh of storage, and 8 kWh of shifted consumption needs 8.9. Round-trip efficiency is a real 11% tax on everything a battery does.
One input changed at a time. Every figure is computed by the calculator on this page.
| Situation | Backup need | Shift need | Both |
|---|---|---|---|
| Base case2 kW × 12 h, 8 kWh shift, 90% RTE | 26.7 kWh | 8.9 kWh | 35.6 kWh |
| 1 kW for 24 hours | 26.7 kWh | 8.9 kWh | 35.6 kWh |
| 3 kW for 8 hours | 26.7 kWh | 8.9 kWh | 35.6 kWh |
| Efficient battery, 95% RTE | 25.3 kWh | 8.4 kWh | 33.7 kWh |
| Less efficient, 80% RTE | 30.0 kWh | 10.0 kWh | 40.0 kWh |
The first three rows are identical, and that is the lesson: only the product of load and hours matters. Halving what you protect doubles how long you can protect it, which is a far cheaper lever than buying more storage.
Ranked. A proposal can change any of these without saying anything untrue, so these are the inputs to check first.
The dominant input and the one entirely within your control. Cutting the protected load from 3 kW to 1.5 kW doubles your runtime for free. This decision is made when the quote is drawn and then physically wired, so it is hard to change later.
Linear with energy. Twelve hours costs twice what six does. Check your utility’s actual reliability before sizing for a multi-day event.
A quiet 11% tax at the 90% default. Between 80% and 95% the requirement moves from 30.0 kWh to 25.3, which is close to a whole extra unit on some products.
The specification that actually determines whether a well pump or an air conditioner will start. Manufacturers publish it inconsistently: we found 185 LRA stated on one product, "up to 48 A LRA" on another, and no figure at all on a third.
The residential code restricts placement, requires not less than three feet between units, sets clearances from doors and windows and imposes construction requirements on the enclosing space. Establish location before capacity.
The bill-shift half of this calculation is worth nothing on a flat tariff. Check the gap between your import rate and export credit before paying for capacity to arbitrage it.
A whole-home backup is a much larger and more expensive proposition than most people intend, and at least one manufacturer guide forbids it outright for its product. Back up circuits, not a building.
A refrigerator cycles rather than running continuously, so its average draw is far below its label. Summing labels produces a battery specification nobody needs.
Manufacturers publish both and they differ. This tool returns usable energy, so compare it against a product’s usable figure or you will undersize.
Energy tells you how long; power tells you whether it starts at all. A battery with ample kilowatt-hours can still fail to start a well pump. Ask for the surge rating in writing and check it against your largest motor load.
You cannot. Energy held in reserve for an outage is energy not available to shift, which is why this tool adds the two requirements rather than taking the larger.
The code restricts where storage may go and requires separation between units. A capacity that needs three units may not have anywhere compliant to put them.
Multiply the backup load you want to carry by the hours you want to carry it, then divide by round-trip efficiency. A 2 kW load for 12 hours at 90% efficiency needs 26.7 usable kilowatt-hours. Add your daily peak import separately if you also want to shift consumption off expensive hours.
Because only the product of load and hours determines energy. Both are 24 kilowatt-hours of delivered energy and both need 26.7 kWh of usable storage at 90% efficiency. This is why reducing what you back up is the cheapest way to extend how long you can back it up.
It is the share of energy put into the battery that comes back out; the rest is lost to heat in charging and discharging. At 90% you need 26.7 kWh of storage to deliver 24 kWh of useful energy. It is a real tax on everything a battery does, and the datasheet gives the figure for your product.
No. Manufacturers publish a nameplate capacity and a usable capacity and they differ, because a battery is not run to absolute empty. This tool returns usable kilowatt-hours, so compare it against the usable figure on the datasheet or you will undersize.
Not necessarily, and this calculator cannot tell you. Starting a motor needs surge power, which is a separate specification from energy. Manufacturers publish it inconsistently, we found one stating 185 LRA, another saying "up to 48 A LRA" and a third publishing nothing. Ask for the figure in writing and check it against your largest motor.
No manufacturer publishes a run time, because it depends entirely on what you connect. What we can tell you from the datasheets is that usable capacity divided by a product’s own maximum continuous output comes to roughly 1.2 to 1.3 hours across the products we read. That is the floor, at full output.
Only by splitting its capacity. Energy held in reserve for an outage is not available to shift, which is why this tool adds the two requirements rather than taking whichever is larger. If you want both fully, you need both amounts.
Check your utility’s actual reliability first. Sizing for a rare event is expensive, and the alternative for very long outages is usually a generator, which brings its own risks, the federal record includes at least 1,332 carbon monoxide deaths between 2004 and 2021. Our comparison page covers that trade honestly.
Yes, substantially. Residential storage must be listed and labelled to a storage standard, individual units must be separated by not less than three feet unless the listing says otherwise, there are clearances from doors and windows, and enclosing walls and ceilings carry construction requirements. Settle the location before the capacity.
That depends on your export credit, not on the battery. Under full retail net metering, storing a kilowatt-hour and exporting it are worth the same and the arbitrage value is zero. Under net billing there is a real gap. Our net billing calculator puts a ceiling on it so you can compare against the quoted cost.
Every assumption in this calculator is argued from primary sources somewhere in our research library. These are the pages that matter for this one.
Nominal 25-year totals for cash (or financed) purchase versus an escalating lease.
What the monthly lease payment becomes in year 25, and the cumulative paid.
Normalise a quote to $/W, unwind a dealer fee, and price year-1 production. Not a market index.
Actual results depend on roof, usage, utility rules and a real proposal. Matching is still being built. The form is an enquiry, not a dispatch line.
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Primary-source research, data analysis and fact checking
We are a research desk, not a sales floor. We read the statute, the tariff, the code section, the federal filing or the manufacturer data sheet ourselves, and we publish the figure with the document it came from and the date we retrieved it. Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify. That rule has cost us whole sections, and it is the reason the rest can be trusted.
How this desk works
Data as of 23 August 2026. Authorship on this site is organisational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.